We're a news company so we have almost 10,000 articles, this is probably the biggest challenge.

The question here is how is currently structured.

For instance i had a client which is a Magazine have made a process to migrate from a custom made CMS, and due the nature of it was a huge process since i could not find a way to migrate directly, so it was needed to entry the articles by hand (since there were encoding problems), and were around 2000 articles on the span of 8 years on the old CMS, (photos of older notes were sacrificed since disappeared). The categories were set, the notes sent to their sections and the archive is working well.

With extensions or plugins specialized on migrating pages and posts is critical to see how are they translated and if everything comes crystal clear or anything gets missing.

Other point to take into account: Current Database Size; while i was in that project in development stage i had to apply some performance settings since the articles volume put the site in a slow load and test were done and performs decently now.

Take into account that due the articles volume you mention it will be an extended process since articles, categories and tags should be verified in order to see if process can be accelerated or extended, since in early stages you need to see with some point (e.g. today), try the post/page converter, see the result and set up the template in function of it and in a later stage export again with current content so template adjustments won't be lost.